Receiving circuit



y 25, 1939- B. D. H. TELLEGE N 2,157,011

RECEIVING CIRCUIT Filed Jan. 7, 1937 Algal INVENTOR BERNARDUS D.H.TELLEGEN ATTRNEY Patented July 25, 1939 UNITED STATES PATENT OFFICE.

lands, assignor to N.

V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands, a corporation of the Netherlands Application January "I, 1937, Serial No. 119,464

In Germany January 13, 1936 2 Claims.

This invention relates to a receiving circuit provided with an amplifierhaving connected to it an electro-mechanical device, for example a loudspeaker or a recording device, and has for 5 its object to suppress the distortion that occurs in such devices as a result of mechanical phenomena occurring in them, such as resonance or the like.

According to the invention, feed-backwhich i is known per se is resorted to for this purpose.

The feed-back voltage consists of two components counteracting -each other, one of which is proportional or substantially proportional to the voltage at the electro-mechanical device and the 1 other is proportional or substantially proportional to the current taken up by the device. If the amplifier has connected to it an electromagnetic or an electrodynamic device, the first component must act in a negative sense, that is to 30 say, an anti-regenerative sense. Ii the device is electrostatic the first component must act in a positive sense. If the electro-mechanical device is connected to the amplifier with the interposition of a so-called output transformer the volt- !5 ages to be fed-back are preferably drawn from impedances arranged on the secondary side of the transformer.

It is well known that a purely electro-mechanical device can be replaced by an electric system comprising an inductance, a capacity and a resistance. The inductance and the capacity of the substitution system are governed not only by the electric values of the coils used or the like, but also by the mass and rigidity respectively,

of the oscillatory system (diaphragm, voice coil,

etc.).

In order that the invention may be clearly understood and readily carried into -eifect it will now be described more fully, by way of example, with reference to the accompanying drawing.

Figure 1 shows by way of example the substitution diagram of an electromagnetic or electro- 5 dynamic loudspeaker system. Figure 2 shows the substitution diagram of an electrostatic device. Figure 3 is a diagrammatic view or a form of construction of an amplifier according 'to the invention and Figure 4 shows a particular arrangement for the circuit of the electromagnetic device.

In Figure 1, R1 designates the resistance and L1 the inductance of the voice coil of the loudspeaker. L2, C and R: are governed by the mechanical values of the system, that is to say by the rigidity, the mass and the damping, respectively.

In Figure 2, C1 designates the natural capacity of the electrostatic loudspeaker. C2, L and R2 are governed as before by the rigidity, the mass .6 and the damping, respectively. In both cases the electric values which replace the mechanical ones constitute an imaginary circuit, the resonance frequency of which corresponds to the natural frequency of the electromagnetic system. '"Due to the fact that according to the invention two voltages are fed back, one of them being substantially proportional to the voltage and the other substantially proportional to the current, the mechanical resonance is damped. This may be readily appreciated when it is taken into consideration that for thepurpose of damping the resonance the voltage would have to be fed back. to L2, C and R: in a negative sense. This voltage cannot be obtained directly but is equal to the voltage at the load less the voltage drop across R1 and L1 (Figure 1). The voltage must be backcoupled in a negative sense and the current in a positive sense.

Figure 2 shows that in contradistinction to this, back-coupling with electrostatic devices must be efiected in the reversed sense. In this case there is a series-resonance circuit and the current that passes through the circuit which includes C2, L and R2, contains the difference between the total current and the current taken up by C1, said current being proportional to the voltage. For the purpose of suppressing the resonance it would be necessary to feed back in a negative sense a voltage which is proportional to the current in the series-circuit. According to the invention, this is ensured by back-coupling in a negative sense, that is to say, in an anti-regenerative sense, a voltage which is proportional to the current, and in a positive sense, a portion of the voltage at the device.

Particularly in the case of resonance nonlinear distortions occur with electro-mechanical devices due to the non-linear variation of several mechanical magnitudes. Even. such distortions are suppressed to a high extent when the back-coupling described is used.

It is possible as hereinafter explained to arrange the circuit in such manner that the backcoupling described is dependent upon the frequency and is active in the sense herelnbefore indicated, particularly adjacent the point of resonance.

Referring to Figure 3, the low-frequency am- 55 plifier is shown comprising two valves I, and I. The low-frequency oscillations to be amplified are supplied to the grid of the valve I through a condenser 2. The anode circuit of the valve I is coupled, by means of a capacity 3, to the grid circuit of the valve I. The anode circuit of the valve I includes the primary of a. transformer 8, the secondary or which has connected to it an electromagnetic device, forexample, an electrodynamic loudspeaker 9.

The grid bias of the valve I depends on .the voltage drop across the resistance 4 which is shunted by a negatively coupling condenser 5.

In addition, there is active in the grid circuit a back-coupled voltage which is supplied by a diagrammatically illustrated device I in which a number of voltages can be combined and which is connected to the resistance 4 by a conductor II. As shown on the drawing the device ID has supplied to it, the current and volta e of the electromagnetic device. The arrangement is such that a portional part of the voltage is retroactively coupled in a negative sense and a voltage proportional to the current is back-coupled in a positive sense. v

A particular form of the device III is shown in Figure 4. The conductor II is connected to the common point of two impedances I2 and I3 connected in series between the secondary transformer terminals. The transformer winding has connected in series with it a third impedance I4,

one point of which is earthed and the arrangeage so fed back is a positive voltage and is applied to the negative end of the cathode biasing resistor 4, the voltage so fed back is applied in a sense to reduce the potential difference between control grid and cathode, or in other words the feed back voltage is applied in degenerative phase. With a proper choice of the magnitude of impedance I4 which is connected in series relation to the voice coil and to impedances I2 and I3 a voltage proportional to the current thru impedance I4 is fed back by way of impedance I3 and the conductor II to the input of tube I. The eflect of the voltage developed across impedance I4 is to increase the negative bias of tube I, in other words, thevoltage is applied in a'regenerative phase.

If it is desired to make the above feed-back voltages dependent upon the frequency, the im pedance I4 may take the form of a parallel resonant circuit tuned to the natural frequency of the loudspeaker, oscillations or vibratory systhe secondary side of the output transformer has.

however, the great advantage that the magnetizing current of the output transformer does not influence the back-coupling and that the combination in the desired phase can be obtained in a particularly simple manner since there is complete liberty in regard to the choice of the point to be earthed.. In addition, when a step-down transformer is used, as is. often the case, the necessary impedances may have comparatively low values since in this case the voltage in the load circuit is compartively low and the current high.

-What I claim is:

1. In a multi-stage vacuum tube amplifier having a ground point and means for impressing signal voltage between said ground and the first tube grid of said amplifier to vary the potential of said grid with respect to ground, said amplifier having its output transformer-coupled to the voice coil of an. electrodynamic loudspeaker,.

means for reducing the efiects of mechanical resonance in said loudspeaker comprising means for developing a voltage proportional to the secondary voltage of said transformer, means for developing a voltage proportional to current through said voice coil, connections for combining said last named voltages to produce a resultant potential variation with respectto ground, and connections for impressing said variation upon the cathode of said first tube, the phase of the first mentioned component of said variation being similar to that of the potential of said grid while the phase of the second mentioned component of said variation is substantially opposite thereto.

2. In a multi- -stage vacuum tube amplifier having a ground point and means for impressing signal voltage between said ground and the first tube grid of said amplifier to vary the potential of said grid with respect to ground, an electrodynamic loudspeaker having its voice coil transformer-coupled to the output of said amplifier, means for reducing the efiects of mechanical resonance in said loudspeaker comprising an impedance connected across the secondary of said output transformer, a .second impedance connected between the low potential ends of said first impedance and the voice coil, and aconductive connection from an intermediate point on saidfirst impedance to the cathode of said firsttube.

,. BERNARDUS D. H. TELLEGEN. 

